Industrial robots are an example of robots whose orientations are set in accordance with changes in the angles of joints. For example, when welding is to be performed by using a welding robot, which is a type of an industrial robot, there is operation called touch sensing for sensing the position of a workpiece to be welded.
Touch sensing is sensing operation that involves moving the welding robot in a state where a voltage is applied to a welding torch and detecting a position where a welding wire of the welding torch comes into contact with the workpiece (that is, a position where an electric current between the workpiece and the welding wire is detected) as a workpiece position. In this touch sensing, the workpiece position is normally detected based on the orientation of the robot (i.e., the angle of a motor for each joint of the robot) when contact with the workpiece is detected.
Patent Literatures 1 to 3 disclose examples of the technology for detecting the workpiece position.
Patent Literature 1 discloses an electrode-position control method in tandem arc welding for controlling the positions of a leading electrode and a trailing electrode relative to a welding line before starting a welding process. The electrode-position control method in tandem arc welding includes: a voltage detecting step involving using current-voltage detecting means to detect voltages of the leading electrode and the trailing electrode brought into contact with a weld workpiece to be welded; a sensing step involving using sensing means to detect positional information about the weld workpiece from electrical changes in the voltages of the leading electrode and the trailing electrode detected in the voltage detecting step; a correction-amount calculating step involving using correction-amount calculating means to calculate a correction amount for correcting positional deviations of the leading electrode and the trailing electrode relative to the welding line, which is preliminarily taught, from the positional information about the weld workpiece detected in the sensing step; a position correcting step involving using robot-trajectory planning means to add or subtract the correction amount calculated in the correction-amount calculating step so as to correct the positions of the leading electrode and the trailing electrode relative to the welding line.
Patent Literature 2 discloses an arc welding apparatus including a robot control device, a welding power source, and an arc sensor control unit that detects an actual welding condition including at least one of a welding current and a welding voltage used during a welding process and that corrects a welding path or a welding condition. The robot control device, the welding power source, and the arc sensor control unit are connected by a bus. At least one of a welding condition required in the welding process and the actual welding condition including at least one of the welding current and the welding voltage used during the welding process is digitally transmitted among the robot control device, the welding power source, and the arc sensor control unit via the bus in accordance with a parallel method. The welding voltage and the welding current are detected by a voltage detector and a current detector provided only in the welding power source.
Patent Literature 3 discloses a welding apparatus including a combination of a robot control device and a control unit of a welding power source. Each of the robot control device and the control unit of the welding power source is of a digital control type and has a digital communication control unit. A welding condition command including at least a welding current command value is transmitted from the robot control device to the control unit of the welding power source and an arc answer is transmitted from the control unit of the welding power source to the robot control device via the communication control unit in accordance with the digital amount.